1. Field of the Invention
The present invention relates to a digital color encoder used in an apparatus such as a game player or a video CD player, for example, equipped with a video output terminal for outputting a composite color video signal capable of being inputted into a television image receiving set.
2. Description of the Related Art
In the case of generating a composite color video signal capable of being inputted into a television image receiving set (TV image receiving set) from three color component video signals (e.g., digital color signals R, G and B) corresponding to three original colors of light (hereinafter, called "original color signal"), an analog system color encoder has conventionally been used. However, in recent years, digital color encoders have been used to generate the above-mentioned composite color video signals by performing digital signal processing on the signals.
FIG. 9 is a block diagram showing a basic structure of such a digital color encoder as mentioned above. Referring to FIG. 9, the conventional digital color encoder is described hereinafter.
The digital color encoder is formed by connecting an input circuit 11, a matrix circuit 12, low-pass filters (LPFs) 13 and 14, a delay circuit 15, a color burst controlling circuit 16, a modulation circuit 20, an adder 24, an addition circuit 25, a synchronization signal adding circuit 26, and a D/A converter 27, as shown in FIG. 9. In such structure, a composite color video signal Stv capable of being inputted into a TV image receiving set is generated from the original digital color signals R, G, and B using a method as described below.
At first, signals are sampled with a predetermined sampling frequency to provide the original color signals R, G, and B which are inputted from the external side by use of the input circuit 11, where the inputted signals are respectively latched. The original color signals R, G, and B thus latched are transmitted to the matrix circuit 12 where they are composed in a known manner so that a brightness (luminance) signal Y and two color difference signals U and V are generated and output.
The color difference signals U and V are limited to a predetermined band width by the low-pass (digital) filters (LPFs) 13 and 14, and a color subcarrier signal of predetermined phase and amplitude is inserted by a predetermined cycle number during a certain period instead of the color difference signals U and V by use of the color burst controlling circuit 16. (Here, the inserted signal is called "a color burst signal"). Thereafter, the color difference signals U and V are inputted into the modulation circuit 20. The modulation circuit 20 comprises a color subcarrier signal generator 21 and multipliers 22 and 23. Two color subcarrier signals having a phase difference of 90.degree. are generated by the color subcarrier signal generator 21 and are respectively balance-modulated with the color difference signals U and V by use of the multipliers 22 and 23.
The two signals thus balance-modulated are added to each other by the adder 24 and inputted into the addition circuit 25 as chrominance signal. In the addition circuit 25, the chrominance signal and the brightness (luminance) signal Y, time-adjusted by the delay circuit 15, are composed. The signal output by addition circuit 25 has a synchronization signal added thereto by the synchronization signal adding circuit 26 to produce a digital composite color video signal.
All of the above-mentioned processings are executed by digital signal processing, and therefore the composite color video signal is a digital signal at this point during the processing. Consequently, the composite color video signal is converted to an analog signal by the D/A converter 27, and the converted signal is outputted as an analog composite color video signal (hereinafter, called "video signal") capable of being inputted into the TV image receiving set.
In the conventional digital color encoder as described above, the sampling frequency of the signal to be inputted to the final-stage D/A converter is the same as that of the original color signals R, G, and B. For example, a frequency of 13.5 MHz is used in accordance with the CCIR (International Radio Consultive Committee) Recommendation as the sampling frequency. However, in this type of system, the following problems may occur:
(1) The high-frequency area component of the video signal is attenuated due to aperture distortion caused by the D/A conversion. PA1 (2) Since the number of sampling points per time period of the transmission color signal is about 3-4, wave form distortion arises in the video signal. In principle, the signal output by D/A converter 27 is transmitted through an analog low-pass filter (LPF) which has a property of removing the upper sideband components contained in the digital composite video signal. That is, the analog LPF has a property of attenuating the high-frequency component not smaller than 1/2 of the sampling frequency so that it is possible to obtain a video signal without any distortion.
However, in practice, phase distortion may occur and the above-described analog LPF cannot be used. Accordingly, wave form distortion occurs in the video signal when the the aforementioned sampling frequency is used. Consequently, there arises a further problem to be solved at the side of the TV image receiving set to which the video signal is inputted. For instance, in a case in which the color burst signal contained in the video signal is distorted, when the standard subcarrier signal necessary for demodulating the chrominance color signal is created from the color burst signal in accordance with a PLL (Phase Locked Loop) method, the created signal is not locked or the locked state cannot be kept on some occasions. Furthermore, some obstacles appear on the image displaying surface of the TV image receiving set due to the wave form distortion of the video signal on some occasions.
On the other hand, it is possible to consider a countermeasure for eliminating the above-described problems, by selecting a higher sampling frequency to start with. However, if the sampling frequency is set to a high value, the amount of consumed electric current increases and circuit design becomes difficult.